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Context: GRB afterglows are excellent probes of gas and dust in star-forminggalaxies at all epochs. It has been posited that dust in the early Universemust be different from dust at lower z. To date two reports directly supportthis contention, one of which is based on the spectral shape of GRB 050904 at z= 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. Weaddress the claimed evidence for unusual (SN-origin) dust in its host galaxy bysimultaneously examining the X-ray and optical/NIR spectrophotometric data.Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunnphotometric data, the spectroscopic and z'-band photometric data at ~3 daysfrom the Subaru telescope, as well as the critical UKIRT Z-band photometry at0.47 days, upon which the claim of dust detection largely relies. Results: Wefind no evidence of dust extinction in the SED. We compute flux densities atlambda_rest = 1250 AA directly from the observed counts at all epochs. In theearliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3+- 0.2 mag) than previously reported and statistically insignificant (<1.5sigma). Furthermore we find that the photometry of this band is unstable anddifficult to calibrate. Conclusions: From the afterglow SED we demonstrate thatthere is no evidence for dust extinction -- the SED at all times can bereproduced without dust, and at 1.25 days in particular, significant extinctioncan be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-typeextinction curve. We conclude that there is no evidence of any extinction inthe afterglow of GRB 050904 and that the presence of SN-origin dust in the hostof GRB 050904 must be viewed skeptically. [abridged]

No evidence for dust extinction in GRB 050904 at z ~ 6.3